Mirroring is a form of data backup. In mirroring, data written to a primary storage volume is also written to a secondary storage volume. The volumes may be logical storage volumes of the same or different storage devices, like hard disk drives. In synchronous mirroring, data written to the primary storage volume is simultaneously written to the secondary storage volume. The data is not considered as having been written to the primary storage volume unless it also has been written to the secondary storage volume.
By comparison, in asynchronous mirroring, data written to the secondary storage volume is written at some point in time after the data has been written to the primary storage volume. The data is considered as having been written to the primary storage volume even before it has been written to the secondary storage volume. Asynchronous mirroring permits the secondary storage volume to be located at a significant distance away from the primary storage volume, without incurring the performance penalty that results with synchronous mirroring where the volumes are located at a significant distance away from each other.